Heating water above 100° C. at 1 atmosphere will transform it into steam. In steam generating devices, such as steam irons, water is applied to a hot surface in order to generate the steam. However, the steam can form an insulating layer between the surface and the water droplets, thereby effectively slowing down the evaporation of water. The water droplets will tend to bounce on the surface instead of evaporating into steam. This effect is called the Leidenfrost effect and generally occurs above 160° C. This effect is for instance observed in steam irons.
Various methods have been proposed to prevent the Leidenfrost effect, ranging from providing special structures in the steam chamber, like ribs for instance, to the use of coatings on the surface of the steam chamber. A suitable steam promoter coating is hydrophilic and moderately heat-insulating. The moderately heat-insulating character of the coating prevents the water from touching the hot aluminum substrate. When some water touches the surface, the surface is immediately cooled down effectively to below Leidenfrost effect temperatures. Preferably also, such steam promoter coatings do have a certain amount of porosity. By virtue of the hydrophilic character of the steam promoter coating, the water introduced spreads readily over the surface of the steam chamber. A suitable steam promoter coating offers a combination of good wetting, absorption of water into the porous structure, and a high surface roughness.
A steam generating device of the type described in the preamble is known from U.S. Pat. No. 5,060,406. The known device (a steam iron) is provided with a steam promoter coating, mainly composed of silica, fillers and an acid phosphate compound, in particular mono aluminum phosphate. Due to the presence of a relatively large amount of fillers, the known coating mixtures are highly viscous and cannot be readily applied by spraying techniques. Mono aluminum phosphate is water-soluble, can be dried and cured into a substantially insoluble inorganic coating, and also acts as acid stabilizer for the colloidal mixtures of silica, used in U.S. Pat. No. 5,060,406. Moreover, it has a low pH and therefore etches the aluminum substrate, which improves the adhesion between the coating and the aluminum substrate. It would therefore be desirable if a steam promoter coating, based largely on mono aluminum phosphates alone, could be applied to the steam chamber surface. However, such coatings are too brittle, and can therefore be applied in small thicknesses only, typically less than one micron. This is not desirable, inter alia, because such small thicknesses increase the risk of occurrence of the Leidenfrost effect.
In summary, the known steam promoter coating does reduce the Leidenfrost effect to the desired level, but is either too viscous or too brittle, in particular, in an environment having a high degree of humidity and a relatively high temperature. This brittleness causes flakes to break away from the steam chamber coating, and said flakes can leave the iron through the steam ports.